1. Field of the Invention
The present invention relates to a photosensitive material where a character or a mark is printed by irradiation of a laser beam and a laser marking method thereof, in particular relating to the laser marking method where a dot pattern is formed at a certain position on the photosensitive material while the laser beam is irradiated from a laser onto the photosensitive material in which an emulsion layer is provided on a surface of a supporting body, a marking pattern including the visible character or mark is formed by a combination of the dot patterns.
2. Description of the Related Art
As described in Japanese Patent No. 3191201, a marking method where a laser beam is irradiated intermittently to form a dot pattern in order on a surface of a photosensitive material, and a character or a mark being readable by eye is printed by a range of the dot patterns is known. A partially melted trace is remained on the surface of the photosensitive material by irradiation of the laser beam, the melted trace is observed as the dot pattern, it is necessary that each dot pattern is easily distinguishable, in order to recognize with accuracy the character or the sign represented by an array of the dot patterns.
In order to form the easily distinguishable dot pattern, in the method described in the above-described gazette, some conditions such as energy density of the laser beam and irradiation time (pulse width) of the laser beam are set. However, except the irradiation time of the laser beam, since it is difficult to measure the energy density of the laser beam, it is actually difficult to manage the energy density of the laser beam, when a kind of the photosensitive material is varied, composition of a base layer or an emulsion layer is varied, so that this method is not an actual technique from a point of view of forming the dot pattern having stable quality.
When the laser beam is irradiated from an emulsion layer side of the photosensitive material, in case that the laser beam is reached to the base layer with a certain intensity, impurities contained in the base layer is emitted by acceptance of the irradiation, so that it is confirmed that a fog is generated in the emulsion layer. When the defect of the fog is generated, quality of the photosensitive material is remarkably reduced.
As shown in Japanese Patent No. 3191201, a marking method where the laser beam having a beam spot of an almost circular shape is irradiated to print the character or the mark while the photosensitive material in a shape of a web is conveyed is known. By irradiation of the laser beam, the dot pattern corresponding to the energy density and the irradiation time (pulse width) is formed on the surface of the photosensitive material, the dot patterns can be arrayed by proper number of dots in column and proper number of dots in row to print the optional character or mark.
In the method described in Japanese Patent No. 3191201, a technique where a plurality of laser oscillators are arranged in parallel in a scanning direction perpendicular to a conveying direction of the photosensitive material, the laser oscillators are selectively turned on and off corresponding to the character pattern to be printed is taken, the conditions of the energy density of the laser beam and the irradiation pulse width of the laser beam are set so as to form the dot patterns uniformly by each laser oscillator.
In case that the energy density and the pulse width of the laser beam irradiated to the photosensitive material can be controlled accurately as Japanese Patent No. 3191201, the dot pattern becomes uniform to increase the visibility of the character pattern. However, the pulse width per dot of the laser beam is an order from 10−5 to 10−6 sec, it is very difficult to measure accurately the energy density of the laser beam in the pulse width. Though it is possible that the laser beam is continuously oscillated and mean energy density is estimated from a diameter of the printed dot, because there is difference between output power characteristics in case of continuous oscillation of the laser oscillator and the output power characteristics in case of build up driving, estimation of the energy density within the pulse width is not reliable.
In the irradiation spot of the laser beam, generally the energy density of a peripheral portion of the spot is lower than that of a central portion. Accordingly, in case that the laser beam is irradiated at a shorter pulse width, the dot pattern having the smaller diameter than the irradiation spot is formed, in case that the laser beam is irradiated at a longer pulse width, the dot pattern having the larger diameter than the irradiation spot is formed because heat conducts surroundings of the irradiation spot. Even though the energy density of the laser beam is calculated based on the dot diameter, the former is calculated higher than actual and the latter is calculated lower than actual.
For the reason, except the pulse width of the laser beam, it is not practical that condition for printing on the photosensitive material is set on a basis of the energy density. Furthermore, when a plurality of laser oscillators are arranged in parallel to use, it is necessary that conditions of each laser oscillator are set, and it is necessary to correspond change in kinds of the photosensitive material, so that it is not practical from a point of view of uniformity of the dot pattern constituting the character and the mark. When the dot pattern is dispersed, a dot having little concave and convex portions and low visibility is generated, energy is given to the impurities by excess laser beam irradiation and the fog of the photosensitive layer is generated by emission.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 10-305377, a detecting method for a laser forming state and a laser forming system are disclosed as a technique marking a character or a sign on a surface of a material by using a laser beam.
In JP-A No. 10-305377, when a part of layers of a laminated film, which is combined by resin films having different laser absorption characteristics, is formed by a laser, for example, half-cutting forming (melting only some of layers) for giving easy opening to a packaging is described.
A technique that the laser beam is irradiated to the photosensitive material such as an x-ray film to form a marking by thermal fogging or deformation of a surface of the photosensitive material is proposed in Japanese Patent No. 3191201.
In Japanese Patent No. 3191201, irradiation time (pulse width) of the laser per dot is set at least not lower than 30 μm in order to form the thermal fogging and the deformation for increasing visibility.
However, when the photosensitive material is conveyed at high speed, a problem that the irradiation time of the laser cannot be taken sufficiently is generated.
As shown in FIGS. 24A and 24B, when one character is formed by a dot matrix of 5×5 dots (=25 dots) (in case of FIG. 24A, all dots are represented by solid), an area of one character should be 1.8 mm and a dot diameter should be 200 μm, actually the irradiation time is not sufficient, which causes the dot to be flown. A case that the dot is flown by 100 μm is shown in FIG. 24B, as a result, the pattern is formed in only an area (overlapping area) always irradiated by the laser while the dot is flown.
That is to say, there is a problem that, when the conveying speed of the photosensitive material is increased, the proper dot pattern is not formed by the irradiation time in the present and the conveying speed is limited.
Suppose that the dot is flown by 100 μm is a tolerance, relation between the irradiation time and the conveying speed of the photosensitive material, as shown in FIG. 25, limits to 200 m/min in a related art of 30 μm.
For example, in a system which the photosensitive material is conveyed in a sub-scanning direction while the laser beam is scanned in a main-scanning direction, when one character is formed by five dots in the main-scanning direction (laser scanning direction) and five dots in the sub-scanning direction (conveying direction of the photosensitive material) (5×5=25), an area of one character is 1.8 mm, irradiation possible time t per dot can be represented by the following equation.
Irradiation possible time per dot=scanning width of one character/(line speed×the number of dots of one character).
For example, let the present line speed be 200 m/min,t=(1.8 mm)/{(200 m/min)×25}Arranging the unit,t=(1.8×1000×60000)/(200×1000×25)=108000000/5000000=21.6 μsec
As shown in the above-described calculation, the necessary irradiation time of the laser is not obtained. Furthermore, when the dot matrix of two characters is formed by one main-scan, only 10.8 μsec of a half value of the calculation can be secured.
On the contrary, when the dot matrix of two characters is formed by 30 μsec of the related art, the line speed is limited to 80 m/min. In case of the dot matrix of one character, the line speed is limited to 160 m/min.
For this reason, the number of dots of one character constituting the dot matrix has to be reduced in order to increase the line speed faster than the speed described above, which results in reduction of the visibility, and in worst case, it is possible that the character and the sign are not distinguishable. The laser intensity is set at tolerance limit, the laser intensity is not increased any more because the fog is generated by emission of the dust in the emulsion layer.